Known electric power steering devices are disclosed in Japanese Patent Laid-Open Publication No. 2001-278084 and Japanese Patent Laid-Open Publication No. H02-290782. The known electric power steering devices include a neutral angle estimation means for estimating a neutral angle θc of a steering angle based on a sampling value θa of the steering angle when a predetermined sampling condition is established. The predetermined sampling condition is defined by a vehicle speed V, an angular velocity ω of the steering angle, a steering torque T, or relating values of the vehicle speed V, the angular velocity ω of the steering angle, and the steering torque T.
FIG. 9 shows transaction of a neutral estimation calculation portion (i.e., the neutral angle estimation means) of the known electric power steering device. With Steps 710-730, whether the predetermined sampling condition defined by the vehicle speed V, the angular velocity to of the steering angle, and the steering torque T is established is examined. V0, T0, and ω0 show predetermined boundary values for defining each boundary condition. When all boundary conditions are satisfied, it is estimated that the steering wheel is positioned approximate to the neutral point. In this case, in Step 770, the value of the neutral angle θc of the steering angle is renewed based on the sampling value θa of the steering angle when the sampling conditions are established. The neutral angle θc on the left side of the equation in Step 770 shows the value which is to be presently obtained and is calculated and renewed every cycle based on last obtained value (i.e., the value of neutral angle θc on the right side of the equation of Step 770).
In this case, a parameter λ includes a predetermined forget factor or a weighting factor depending on the number n of all samples (e.g., (n−1)/n). With the known electric power steering devices, the absolute angle θ (=θa−θc) referring to the neutral point (i.e., the neutral angle θc) is calculated following the foregoing transaction.
For example, with the known devices, the optimization of the parameter λ (i.e., tuning) is not easy and the known devices have following drawbacks.
(1) Problem of Convergence
In order to obtain the neutral angle θc with high precision, it is required to collect large number of samples (i.e., sampling value θa). Thus, the estimated value of the neutral angle θc is unlikely converged to a predetermined value in a short period.
Normally, the value of the neutral angle θc memorized during last engine stop is applied as an initial value (i.e., a temporary value) for a present operation. When an error of the initial value is increased due to the steering wheel operation under a state that the engine is stopped, relatively long time is required to converge the estimated value of the neutral angle θc to the predetermined value unless the forget factor (λ) is relatively small. However, when the forget factor (λ) is determined to be an unnecessarily small value, the estimated value is unnecessarily influenced by the most updated sampling value θa to deteriorate the stability of the estimated value.
(2) Problem of the Reliability
On the other hand, even when the estimated value of the neutral angle θc is converged to be a correct value, the position of the correct neutral point (i.e., correct value of the neutral angle θc) may be varied due to the disturbance generated relative to the steering system thereafter. In other words, there is no guarantee that the estimated value of the neutral angle θc is always correct not only at the engine start but also during the normal vehicle driving. Thus, with the known electric power steering devices, it is unlikely to achieve the optimum steering wheel returning control always following the reliability of the estimated value.
A need thus exists for an electric power steering wheel device which includes a means for converging an estimated value of a neutral angle θc in a short period with high precision and a means for quantitatively evaluating an accuracy (i.e., reliability) of the estimated value of the neutral angle θc with high precision.